Welding head and method for joining a workpiece

ABSTRACT

A method for joining a workpiece by means of a welding head, in which at least one line of light is generated on a workpiece, which crosses a joint line at a site to be joined and a joint seam generated at a joined site after processing. The lines of light are imaged at the site to be joined and at the joined site in order to generate reference data relating to the geometry of the site to be joined and measurement data relating to the geometry of the joined sites. The reference data and the measurement data are then compared at one and the same workpiece site before and after processing, in order to determine the geometry of the joint seam independently of the geometry of the site to be joined.

The invention relates to a welding head, in particular a laser weldinghead, and to a method for joining a workpiece, in particular by means ofa laser beam, by welding or by soldering.

With the aid of a laser welding head, a workpiece can be processed byusing a laser beam, in which case, for example, welding or solderingwork may be carried out in order to join a gap in a workpiece or betweentwo workpieces. In this process, it is necessary to monitor the qualityof the weld or solder seams produced by the laser welding head by meansof the laser beam. The inspection of the weld or solder seams is carriedout by means of image processing, the geometrical properties of the weldseams such as concavity, convexity, seam width or seam thickness interalia being determined. In order to record these properties, the seamregion must be known exactly in the three-dimensional representation,since otherwise irregularities in the workpiece in the region of theseam to be joined will also possibly be included when recording thegeometry of the joint seam. The welding process described above is not,however, restricted to laser welding, and for example welding by meansof a metal shielding gas welding head likewise requires weld seammonitoring.

The most common evaluation of the three-dimensional geometricalstructure of joint sites is carried out by means of laser triangulation.In this method, during a welding process, a light section device is usedwhich is attached to the laser welding or metal shielding gas weldinghead. The light section device projects a light fan by means of a laserbeam onto the workpiece, in order to generate a line of light thereon.From observation of the light section, i.e. the line of light, thegeometries of the site to be joined and the joined site after processingby means of the laser beam, i.e. the weld or solder seam, can bedetermined by means of the shape of the line of light in the processingregion.

In known methods for determining the geometry of the joint site forquality monitoring, optionally the sheet metal geometry without a weldor solder seam is stored beforehand and subsequently compared with themeasurement data after joining the workpiece. This, however, eitherrequires full knowledge of the sheet metal geometry without a weld seambeforehand or conduct of a reference run before the welding process, inorder to record the geometrical data of the joint site without a weldseam. During the inspection, these data are compared with the currentmeasurement data and discrepancies are thus identified. In this method,however, in the event of path changes between the reference andmeasurement runs, elaborate manual modification of the reference data isnecessary. Furthermore, component deviations and modifications of thecomponent during the joining process are not able to be recorded andtherefore lead to measurement errors. Furthermore, modification of thecomponent position due to clamping devices is not taken into account.

US 2005/02 47 681 A1 describes a welding head, which comprises a housingthrough which a beam path for a laser beam is formed. The housingcomprises focusing optics, two light section devices which generate twolines of light extending parallel, two CMOS cameras and a processingunit, which is used to observe a joint gap and a weld seam and tomonitor a position of the welding head relative to the weld seam.

WO 2008/028 580 A1 describes a method and a device for opticalassessment of the weld quality in welding. During the laser welding, thewelding region is imaged coaxially to the laser beam through the laseroptics, both a triangulation line and a gray or color image of thesolidified weld seam, as well as the process light of the weldingprocess, being recorded. Optimal quality assessment of the weldingprocess and the weld seam can be carried out from these three imageelements.

DE 10 2006 004 919 A1 describes a laser beam welding head. This laserbeam welding head for welding metal parts comprises at least one beampath for a welding beam and means for optically recording the positionof the weld seam at a first measurement position, the means foroptically recording the position of the weld seam allowing arrangementof the first measurement position running in front of the weldingposition of the welding beam in the welding direction, and generating acorrection signal for correcting the welding position of the weldingbeam at least as a function of a lateral deviation of the weld seam froma setpoint position, and a corresponding use of the laser beam weldinghead.

EP 2 062 674 A1 describes a method of preparing for and carrying out alaser welding process. This method of preparing for a laser weldingprocess on a workpiece comprises the steps: recording the position of ajoint site on the workpiece with the aid of a sensor device in a firstmeasurement region running in front of a laser beam position, recordingthe position of the joint site with the sensor device in a secondmeasurement region at the laser beam position and/or in a thirdmeasurement region running behind the laser beam position, recording thelaser beam position in the second measurement region with the sensordevice, and comparing the positions of the joint site in the respectivemeasurement regions and the laser beam position in order to adapt theposition, alignment and/or coordinate system of the sensor device and/orof a laser processing head relative to the workpiece.

DE 10 2007 030 395 A1 describes a method and a device for laser beamwelding of a workpiece. This method is provided for preferablycontinuous laser beam welding of a workpiece, in particular a tube,along a welding direction along the workpiece, at least one markingoffset with respect to a joint gap in the workpiece being detected onthe workpiece in front of a welding site in the welding direction. Theat least one marking is also detected behind the welding site in thewelding direction, and an optimal welding position of the laser beamtransversely to the welding direction is determined from the positiontransversely to the welding direction of the marking detected in frontof and behind the welding site.

EP 0 770 445 A2 describes a method for controlling and positioning abeam for processing workpieces. In a method for controlling andpositioning a beam for processing workpieces, a first sensor in front ofthe beam or a specification determines the path to be tracked by thebeam. A second sensor behind the beam monitors the activity of the beam.The specification or the recording results of the first sensor relatingto a setpoint position of the beam are compared with recording resultsof the second sensor relating to an actual position of the beam, whiletaking into account the speed-dependent relative beam/workpiecedisplacement. In the event of a difference of the actual position fromthe setpoint position, the beam is corrected to a base position.

It is an object of the invention to provide a welding head and a methodfor joining a workpiece, by which quality monitoring of a joint seam canbe carried out easily during an ongoing joining process.

This object is achieved by the welding head as claimed in claim 1 and bythe method for joining a workpiece as claimed in claim 15. Advantageousconfigurations and refinements of the invention are presented in thedependent claims.

The invention provides a welding head for joining a workpiece, whichcomprises a welding device which is adapted to weld a joint site of theworkpiece to be processed inside a working region, a light sectiondevice which is attached to the welding device, can be rigidly connectedto the housing and has at least one light source for generating at leastone line of light inside the working region on the workpiece, whichcrosses a site to be joined and a site joined after processing by thewelding device at a predetermined distance, at least one camera forobserving the working region of the workpiece to be processed, whichimages the line of light in or at the site to be joined and the line oflight in or at the joined site at regular time intervals, in order togenerate reference data relating to the geometry of the site to bejoined and measurement data relating to the geometry of the joined sitewith a joint seam, and a processing unit for receiving the referencedata and measurement data from the at least one camera and for comparingthe reference data and measurement data respectively at the sameworkpiece site before and after processing by the laser beam, so thatthe geometry of the joint seam can be determined independently of thegeometry of the site to be joined.

A welding head for joining by means of welding or soldering is thusprovided, in which the monitoring of the weld or solder seam is carriedout by recording the geometry of the seam to be joined by means of aline of light running in front and comparing these recorded data withthe data recorded by means of a line of light running behind, whichimages the joined seam. In this case, a processing unit is provided inthe welding head, which compares the recorded geometrical data beforethe joining process and after the joining process so that the data atthe same respective workpiece site can be compared with one another.According to the invention, this may be done by determining the joiningdisplacement traveled on the basis of integrating a known joining speed,which corresponds to the speed of the welding head or the speed of theat least one camera which is rigidly connected to the welding head, thejoining displacement traveled being compared with the knownpredetermined distance between the line of light running in front andthe line of light running behind, so that a time difference between thereference data and the measurement data can be calculated. From thecomparison of the reference data, the geometry of the joint seam cantherefore be determined independently of irregularities in the workpieceto be joined in the region of the seam to be joined, and qualitymonitoring can be carried out in an ongoing joining process.

In one configuration according to the invention, it is particularlyadvantageous for the welding device to be a metal shielding gas weldinghead.

In this case, it is particularly expedient for the at least one camerato be attached to an outer side of the welding device. In the case ofmetal shielding gas welding, it may be expedient to attach two camerasto the outer side of the welding device, since the line of light runningin front and the line of light running behind cannot be recorded bymeans of one camera owing to obstruction by the device. In this case,the recorded images of the two cameras are correlated with one anotherso that corresponding reference data and measurement data can begenerated, as is the case when recording by means of one camera.

In another advantageous configuration, it is expedient for the weldinghead according to the invention to be a welding head or laser weldinghead, the welding device comprising a housing, through which a beam pathfor a laser beam is formed and which has focusing optics for focusingthe laser beam onto the joint site of the workpiece to be processedinside the working region. In this case, the light section device havingat least one light source for generating at least one line of lightinside the working region on the workpiece, which crosses a site to bejoined and a site joined after processing by the laser beam at apredetermined distance, is attached to the housing.

For operation of the processing unit, to determine a corrected geometryof the joint seam from the reference data and the measurement data, itis advantageous for the processing unit to comprise a buffer memory fortemporarily storing the received reference data, a comparator forcomparing the measurement data at a respective first instant with thereference data at a respective second instant, the respective first andsecond instants respectively having a predetermined time difference, andan integrator for determining the respective predetermined timedifference by means of integration of the joining speed with respect totime and comparing the calculated joining displacement with thepredetermined distance between the lines of light.

Although in principle it is conceivable for the light section device tobe suitable for projecting an annular line of light onto the workpiece,according to a particularly simple configuration of the invention thelight section device comprises a first light fan device having a firstlight source for generating a straight line of light, which crosses thesite to be joined, and a second light fan device having a second lightsource for generating a straight line of light on the workpiece, whichcrosses the joined site.

For particularly simple evaluation of the reference data and themeasurement data and for simple determination of the predetermineddistance between the site to be joined and the joined site at the lineof light crossing point, it is expedient for the straight lines of lightof the first and second light fan devices, which are generated on theworkpiece, to run parallel to one another.

In order to be able to determine the distance between the weldingdevice, in particular the focusing optics, and the workpiece easily bymeans of triangulation, it is expedient for the first and second lightfan devices to be arranged with respect to one another so that the lightfan of the first light source and the light fan of the second lightsource respectively strike the workpiece to be processed obliquely withrespect to the optical axis of the laser beam.

For producing a small distance between the parallel lines of light inthe case of light fan devices mounted rigidly on the welding head, it isparticularly expedient for the light fans of the first light source andof the second light source to be arranged with respect to one another sothat they converge with one another starting from the respective lightsources.

For the function of processing the reference data and the measurementdata, a constant distance between the lines of light is indispensable.It is therefore particularly expedient for the welding head furthermoreto have a control unit which regulates the distance between the weldingdevice, in particular the focusing optics, and the workpiece to aconstant value by determining the distance between the mutually parallellines of light of the first and second light fan devices.

In a particularly expedient configuration of the welding head, the atleast one camera is a CMOS camera.

For a particularly compact configuration of the welding head, it isexpedient for the welding head furthermore to have a beam splitter bywhich an observation beam path of the at least one camera, in the formof a single camera, can be coupled coaxially into the laser beam path.The use of merely one camera is preferred owing to the lower costs andowing to the simpler calculation of the reference data and themeasurement data.

Owing to the high intensity and the small beam broadening of laserlight, it is advantageous for the first and second light sources to belasers, in particular semiconductor lasers.

In order to eliminate interfering radiation, such as occurs for exampleduring operation of the welding head, it is expedient for an opticalbandpass filter, which is tuned to the wavelengths of the first andsecond light sources, to be arranged in front of the at least onecamera.

The invention furthermore provides a method for joining a workpiece bymeans of the welding head according to the invention, in particular by alaser beam, which comprises the following steps: generating at least oneline of light inside a working region of a workpiece, which crosses asite to be joined and a site joined after processing by the weldingdevice at a predetermined distance, imaging the lines of light in or atthe site to be joined and in or at the joined site at regular timeintervals by means of at least one camera, in order to generatereference data relating to the geometry of the site to be joined andmeasurement data relating to the geometry of the joined sites, andprocessing the reference data and measurement data generated by the atleast one camera by means of a processing unit, the processingcomprising the comparison of the reference data and measurement datarespectively at one and the same workpiece site before and afterprocessing by the laser beam, in order to determine the geometry of thejoint seam independently of the geometry of the site to be joined.

Expediently, the processing step furthermore comprises the temporarystorage of the received reference data; comparison of the measurementdata at a respective first instant with the reference data at arespective second instant, the respective first and second instantsrespectively having a predetermined time difference; and determining therespective predetermined time difference by means of integration of thejoining speed with respect to time and comparison of the calculatedjoining displacement with the predetermined distance between the linesof light.

In order to ensure a constant distance between the lines of light on theworkpiece in the region of the site to be joined and of the joined site,it is furthermore advantageous for the method to comprise the step ofregulating the distance between the welding device, in particular thefocusing optics, and the workpiece by means of triangulation.

The invention will be explained in more detail with the aid of thedrawing, in which:

FIG. 1 shows a highly simplified schematic view of a welding headaccording to an exemplary embodiment of the invention,

FIG. 2A shows a highly simplified perspective detail view of theworkpiece during a joining process at a first instant,

FIG. 2B shows a highly simplified perspective detail view of theworkpiece during the joining process at a second instant, and

FIG. 3 shows a block diagram of a processing unit of the welding headaccording to the invention.

In the various figures of the drawing, components which correspond toone another are provided with the same references.

FIG. 1 shows a highly simplified view of a welding head 10, inparticular a laser welding head, according to an exemplary embodiment ofthe invention, in the way it is used with laser processing machines orsystems. A working laser beam 12 coming from the laser processingmachine is directed through a housing 14 of the welding head 10 onto aworkpiece 16 and focused by means of focusing optics 18 onto theworkpiece 16, as indicated by the optical axis L. The working laser beam12 may be broadened, in the case of supply to the welding head 10 bymeans of a light guide fiber, owing to the extraction of the laser beamfrom the light guide fiber by collimator optics.

Instead of a laser welding head, a metal shielding gas welding head mayalso be used as the welding device, in which case two cameras (notshown) are used in order to observe the workpiece. In what follows,however, the invention will be explained with reference to the use ofone camera.

In the housing 14 of the welding head 10, a beam splitter 20 is arrangedin the passage region of the working laser beam 12 so that anobservation beam path (indicated by its optical axis) of a camera 24 iscoupled coaxially into the beam path of the working laser beam 12. Inthe observation beam path 22, imaging optics 26 and an optical bandpassfilter 28 are arranged in front of the camera 24. In the exemplaryembodiment of the invention as shown in FIG. 1, the observation beampath 22 of the camera 24 is directed by means of the beam splitter 20onto a working region of the workpiece 16. It is, however, also possibleto fit the camera 24 with observation optics on an outer side of thehousing 14 of the welding head 10, in which case, however, it isnecessary to ensure that the image of the working region of theworkpiece 16 as recorded by the camera 24 moves synchronously with amovement of the welding head 10 with the housing 14 and in particularwith the focusing optics 18.

Arranged on an outer side of the housing 14 is a light section device 30which comprises a first light fan device 32 and a second light fandevice 34. The first light fan device 32 is mounted by means of asupport 36 on a side of the housing 14 which lies at the front duringmovement of the welding head 10 in its movement direction (indicated bythe arrow A).

The first light fan device 32 comprises a first light source 38, bywhich a light fan 40 is projected in the direction of the workpiece 16,in order to generate a line of light 42 (shown in FIG. 2A) on itssurface inside the working region of the welding head 10.

The second light fan device 34 is mounted by means of a support 44 on aside of the housing 14 of the welding head 10 which lies on a rear sideof the housing 14 during movement of the welding head 10 in the movementdirection A. The second light fan device 34 comprises a second lightsource 46, by which a light fan 48 is projected in the direction of theworkpiece 16, in order to generate a line of light 50 on its surfaceinside the working region of the welding head 10.

As first and second light sources 38, 46 of the first and second lightfan devices 32, 34, respectively, a laser light source is suitable owingto the high intensity and a small intrinsic beam broadening, in whichcase it may be a semiconductor laser diode. For this, for example,AlGaInP laser diodes having multiple quantum well structures may beused, which have an emission maximum in a wavelength range of between635 nm and 670 nm. For example, a laser diode having an emissionwavelength of 658 nm and an emission power of 66 mW may be used. In thiscase, for reduction of the interfering radiation recorded by the camera,the transmission wavelength of the optical bandpass filter 28 may betuned to the wavelength of the first and second light sources 38, 46.

The welding head 10 furthermore comprises a processing unit 52 connectedto the camera 24 and a control unit 54, likewise connected to the camera24, the functions of which will be described in more detail below.

Although the light section device 30 is not restricted to comprising twolight fan devices 32 and 34, but may also be in the form of a singledevice which, for example, projects a conical light fan around the focalpoint of the laser beam 12 onto the workpiece 16 in order to generate acircular or elliptical line of light, according to the invention it isadvantageous for the first light fan device 32 and the second light fandevice 34 respectively to generate light fans 40 and 48 which lie in anemission plane, so that straight lines of light 42 and 50 arerespectively projected onto the surface of the workpiece 16.

The function of the welding head 10 according to the invention will nowbe explained below with the aid of FIGS. 2A and 2B.

In a joining process carried out by the welding head 10, which may be awelding or soldering process, the welding head 10, as shown by the arrowA indicated in FIG. 1 and FIG. 2, is moved with a speed v(t) over aworkpiece 16 to be joined (which may consist of two metal sheets orsimilar elements to be connected together), the focused laser beam 12striking a respective joint site 56 and, owing to the welding process,generating a joint seam 58 which connects together the workpiece partsshown in FIG. 2A.

The line of light 42 of the first light fan device 32 is projected ontothe workpiece 16 so that it runs in front of the focal point of thelaser beam 12, i.e. the respective joint site 56, so that geometricaldata of the site to be joined can be recorded by means of the camera 24which acquires the entire working region including the line of light 42,the joint site 56 and the line of light 50, in order to record referencedata relating to the site 60 to be joined.

In a similar way, the line of light 50 generated by the second light fandevice 34 on the workpiece 16 runs behind the focal point 56 of thelaser beam 12 and crosses an already joined site 62, so that measurementdata can be recorded by the camera 42 relating to the geometry of theweld seam 58.

As shown in FIG. 1 and FIG. 2A, the first light fan device 32 and thesecond light fan device 34 are arranged with respect to one another sothat they generate light fans which respectively strike the workpiece 16to be processed obliquely with respect to the optical axis of the laserbeam 12, so that, during an up and down movement of the housing 14 alongthe optical axis L (see arrow B), the respective projected lines oflight 42 and 50 on the workpiece 16 move to and fro relative to theworking laser beam 12 striking the workpiece 16. In the case shown inFIG. 1 and FIG. 2A, the line of light 42 generated by the first lightfan device 32 and the line of light 50 generated by the second light fandevice (in the case of a plane surface of the workpiece 16) extendmutually parallel to one another, the light fans of the first and secondlight fan devices 32, 34 converging with one another. A distance dbetween the lines of light 42 and 50 therefore increases when thewelding head 10 is moved downward and the distance d between the linesof light 42, 50 decreases when the welding head 10 is moved upward.

Since, for an optimal joining process, the focus of the working laserbeam 12 should always extend at a predetermined height along the sitesto be joined, the distance d between the lines of light recorded by thecamera 24 is evaluated by the control unit 54 (FIG. 1) and, bycontrolling an actuator (not shown) for an upward or downward movementof the housing 14 (see arrow B), is regulated to a predetermineddistance d which in turn corresponds to an optimal focal position of theworking laser beam 12 on the joint site 56.

Thus, a constant predetermined distance d between the line of light 42of the first light fan device 32 and the line of light 50 of the secondlight fan device 34 can be maintained by the control unit 54 during thejoining process.

The method according to the invention for quality monitoring of thejoint seam 58 will now be explained below with the aid of FIGS. 2A, 2Band 3.

FIG. 2A shows the lines 42 and 50 projected onto the workpiece 16 at aninstant t₁. On the basis of the line shape, the lines of light 42 and 50respectively at the site 60 to be joined and the joined site 62, whichare imaged by the camera 24 at regular time intervals, provideinformation about the geometry or the height profile of the respectivesites 60 to be joined or the respective joined sites 62 at correspondingdiscrete instants throughout the joining process. The object of themonitoring method according to the invention is in this case todetermine geometrical data of the joined site 62 independently of thegeometry of the site 60 to be joined, so that only lines of light whichare located at the same workpiece site (before and after the joiningprocess) are respectively compared with one another for matching orbalancing.

This object is achieved according to the invention in that, as shown inFIG. 2A, the line of light 42 is first recorded by the camera at aninstant t₁ and these data are stored as reference data. At an instant t₂(FIG. 2B), at which the line of light 50 running behind the laserprocessing beam 12 has moved forward by the predetermined distance dbecause of the joining speed v(t), the line of light 50 is recorded andthe geometrical data of the seam now joined are stored as measurementdata. The reference data for the instant t₁ are then compared with themeasurement data for the instant t₂, which relate to the same workpiecesite.

According to the invention, this is achieved by the processing unit 52(FIG. 1), the block diagram of which is shown in FIG. 3.

The processing unit 52 receives at an instant t₁ image data of the linesof light 42, 50 from the camera 24, from which reference data DataR(t)relating to the site 60 to be joined are determined by means of the lineof light 42 and measurement data DataM(t) relating to the geometry ofthe joined site 62 are determined by means of the line of light 50, i.e.the corresponding light sections.

In order to permit comparison of the reference data DataR(t) relating tothe site 60 to be joined with corresponding measurement data DataM(t) atthe same workpiece site, the reference data DataR(t) are first loadedinto a buffer memory 64 in which the reference data DataR(t) can betemporarily stored over a particular period of time. The processing unit52 furthermore comprises an integrator 66, which receives from thewelding head 10 the current joining speed v(t) (which may also beconstant) and which determines the associated joining displacement thathas been traveled by integrating the joining speed v(t) with respect totime. By comparing the joining displacement traveled with thepredetermined distance d, the integrator 66 can thus determine the timedifference Δt by which the reference data DataR(t) and the currentmeasurement data DataM(t) are mutually shifted in time, so that acomparison of the temporally offset reference data DataR(t-Δt) and thecurrent measurement data DataM(t) corresponds to a comparison ofmeasurement data and reference data at the same workpiece site.

This instant is illustrated in FIG. 2B. After the time Δt has elapsed,because of the joining speed v(t) the line of light 50 has traveledforward by the predetermined distance d in the movement direction of thewelding head 10, so that now at the instant t₂ it is at a site of theworkpiece 16 where the line of light 42 was at the instant t₁ (FIG. 2A).By integrating the joining speed v(t) with respect to time and comparingthe displacement traveled with the predetermined distance d, measurementdata and reference data can therefore be determined at the sameworkpiece site.

Furthermore, in addition, by recording the position and orientation ofthe welding head 10 relative to the workpiece 16 (for example bydetermining the path data of a robot arm which carries the welding head10), it is possible to correct errors which result from the projectionof the lines of light 42, 50 onto the workpiece 16 starting from awelding head 10, the beam axis L of which is not perpendicular to theworkpiece surface.

As shown in FIG. 3, geometrical data DataC(t) of the joint seam 58 atcorresponding instants (which may be discrete) are determined from thecomparison of the reference data DataR(t-Δt) and the measurement dataDataM(t), which are independent of the geometry of the site 60 to bejoined, in a comparator 68.

Thus, by integrating sensors running in front and behind in or on thewelding head 10, reference data relating to the geometry of the partsbefore the welding and measurement data relating to the geometry afterthe welding can be acquired simultaneously, so the matching of thegeometrical data can be carried out during the welding process. Onlinemonitoring of the weld seam being produced is therefore possible duringthe welding process.

The method according to the invention and the welding head according tothe invention therefore have the advantage that a reference run forrecording reference data is obviated, so that a higher measurementaccuracy is achieved and effects on the weld seam analysis due tocomponent tolerances, a clamping device which is used or deformation bythe joining, can be minimized or even eliminated owing to the recordingof reference data directly before the joining process. Furthermore, theinspection sensors can be set up simply.

Thus, even during the process of joining a workpiece having componentvariations, in the event of path inaccuracies of the sensor guidingsystem or in the event of deformations during the joining process ormodifications due to the clamping device, simple quality monitoring ofweld and solder seams of all kinds can thus be carried out in theongoing joining process.

1. A welding head for joining a workpiece, comprising: a welding devicewhich is adapted to weld a joint site of the workpiece to be processedinside a working region, a light section device which is attached to thewelding device and has at least one light source for generating at leastone line of light inside the working region on the workpiece, whichcrosses a joint line at a site to be joined and a joint seam generatedat a joined site after processing by the welding device, at least onecamera for observing the working region of the workpiece to beprocessed, which images the line of light at the site to be joined andthe line of light at the joined site at regular time intervals, in orderto generate reference data (DataR(t)) relating to the geometry of thesite to be joined and measurement data (DataM(t)) relating to thegeometry of the joined site with a joint seam, and a processing unit forreceiving the reference data (DataR(t)) and measurement data (DataM(t))from the at least one camera and for comparing the reference data(DataR(t)) and measurement data (DataM(t)) respectively at the sameworkpiece site before and after processing by the welding device, sothat the geometry of the joint seam can be determined independently ofthe geometry of the site to be joined.
 2. The welding head as claimed inclaim 1, wherein the welding device is a metal shielding gas weldingdevice.
 3. The welding head as claimed in claim 2, wherein the at leastone camera is attached to an outer side of the welding device.
 4. Thewelding head as claimed in claim 1, wherein the welding device comprisesa housing, through which a beam path for a laser beam is formed andwhich has focusing optics for focusing the laser beam onto the jointsite of the workpiece to be processed inside the working region.
 5. Thewelding head as claimed claim 1, wherein the processing unit comprisesthe following: a buffer memory for temporarily storing the receivedreference data; a comparator for comparing the measurement data(DataM(t)) at a respective first instant (t₁) with the reference data(DataR(t)) at a respective second instant (t₂), the respective first(t₁) and second (t₂) instants respectively having a predetermined timedifference (Δt); and an integrator for determining the respectivepredetermined time difference (Δt) by means of integration of thejoining speed (v(t)) with respect to time and comparing the calculatedjoining displacement with the predetermined distance (d) between theline of light sections.
 6. The welding head as claimed in claim 1,wherein the light section device comprises a first light fan devicehaving a first light source for generating a straight line of light,which crosses the joint line at the site to be joined, and a secondlight fan device having a second light source for generating a straightline of light on the workpiece, which crosses the joint seam at thejoined site.
 7. The welding head as claimed in claim 6, wherein thestraight lines of light of the first and second light fan devices, whichare generated on the workpiece, extend mutually parallel.
 8. The weldinghead as claimed in claim 6, wherein the first and second light fandevices are arranged with respect to one another so that the light fanof the first light source and the light fan of the second light sourcerespectively strike the workpiece to be processed obliquely with respectto the optical axis (L) of the laser beam, so that a distance betweenthe welding device and the workpiece can be determined by means oftriangulation.
 9. The welding head as claimed in claim 8, wherein thelight fans of the first light source and of the second light source arearranged with respect to one another so that they converge with oneanother starting from the respective light sources.
 10. The welding headas claimed in claim 7, further comprising a control unit which regulatesthe distance between the welding device and the workpiece to a constantvalue by determining the distance (d) between the mutually parallellines of light of the first and second light fan devices.
 11. Thewelding head as claimed in claim 1, wherein the at least one camera is aCMOS camera.
 12. The welding head as claimed in claim 4, having a beamsplitter by which an observation beam path of the camera can be coupledcoaxially into the laser beam path.
 13. The welding head as claimed inclaim 6, wherein the first and second light sources are lasers, inparticular semiconductor lasers.
 14. The welding head as claimed inclaim 6, wherein an optical bandpass filter, which is tuned to thewavelengths of the first and second light sources, is arranged in frontof the at least one camera.
 15. A method for joining a workpiece bymeans of a welding head as claimed in claim 1, having the steps:generating at least one line of light inside a working region of theworkpiece, which crosses a joint line at a site to be joined and a jointseam generated at a joined site after processing by means of the weldingdevice at a predetermined distance (d), imaging the lines of light atthe site to be joined and at the joined site at regular time intervalsby means of at least one camera, in order to generate reference data(DataR(t)) relating to the geometry of the site to be joined andmeasurement data (DataM(t)) relating to the geometry of the joinedsites, and processing the reference data (DataR(t)) and measurement data(DataM(t)) generated by the at least one camera by means of a processingunit, the processing comprising the comparison of the reference data(DataR(t)) and measurement data (DataM(t)) respectively at one and thesame workpiece site before and after processing by the laser beam, inorder to determine the geometry of the joint seam independently of thegeometry of the site to be joined.
 16. The method as claimed in claim15, the processing step further comprising: temporary storage of thereceived reference data (DataR(t)); comparison of the measurement data(DataM(t)) at a respective first instant (t₁) with the reference data(DataR(t)) at a respective second instant (t₂), the respective first(t₁) and second (t₂) instants respectively having a predetermined timedifference (Δt); and determining the respective predetermined timedifference (Δt) by means of integration of the joining speed (v(t)) withrespect to time and comparison of the calculated joining displacementwith the predetermined distance (d) between the line of light sections.17. The method as claimed in claim 15, further comprising the step ofregulating the distance between the welding device and the workpiece bymeans of triangulation.